fix requirements bug (#3798)

# Fixes . path error
* FLARE-2676

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Quick tests passed locally by running `./runtest.sh`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated.

Author: chesterxgchen

examples/advanced/federated-statistics/image_stats/image_stats.ipynb

@@ -33,7 +33,7 @@
    },
    "outputs": [],
    "source": [
-    "%pip install -r image_stats/requirements.txt"
+    "%pip install -r requirements.txt"
    ]
   },
   {